Two similar queries:
- Is there a add-on that will send automatic warning to user if he reached N% of his quota?
- Is there a webmail that shows his current consuption in MB and/or %?
I assume you'e asking about disk quotas implemented by the OS, since that's generally the only type of quota that postfix would have any way of honoring (and would in fact be forced by the OS to honor).
Most mail quotas are not implemented as filesystem quotas in this way. The reason is that filesystem disk quotas are tied to system users, and most mail systems use virtual mail boxes. So the only way to have a per-user quota on a system with virtual mailboxes is to implement the quota in user space. Usually the mail delivery agent (MDA) handles this sort of thing. Postfix has it's own MDA, called 'local', which is the default, but many other MDAs are available. For instance, dovecot, which I use as an IMAP/POP3 server, has its own delivery agent, that (optionally) handles dovecot-enforced quotas.
Now, to actually answer your question, there are Postfix addons that do what you want, to varying degrees of flexibility, and possibly using disk-based or user-space based quotas. I have not used any of them, but have a look here: http://www.postfix.org/addon.html.
To answer your second question, yes, many webmail programs do this--usually with the use of an addon as well. I suggest searching freshmeat, google, or your own Linux distribution's repository for webmail clients, and do your own research to see which supports the features you need.
You should do this research before you implement quotas, because you'll need postfix and your webmail client to understand the same quotas. It won't do any good if postfix obeys the filesystem quotas, but your webmail client is trying to enforce its own user-space quotas.